Spectre + Troll = very fast-hitting Haunt illusions (with max Desolate). Hell on supports. Requires dual lanes, or for Troll to go mid (preferably against a weak enemy mid). You might get away with "support Troll" if no aggro-tri against you.

Faceless Void + Witch Doctor / Jakiro / Lich = very good damage-dealing synergy with Chrono, plus defensive supports to protect a farming 1-position Void. Note: it's far easier to ban out the Void than trying to ban out the multiple supports that are good with Chrono!

Wisp + Tiny/CK/Sven + Nature's Prophet = a very powerful global ganking combination. While you may not be able to prevent the Wisp + Carry, at least don't let them get both Wisp and NP! AA, Clockwerk, and Invoker are also good with this combo.

Lycan + Death Prophet = painful pushing power. Lycan runs around the back to demolish supports, while Exorcism goes to work on the rest of the team.

Visage = heralds an aggro tri-lane (usually). May also prompt a Drow last-pick.

Vengeful Spirit = heralds a roaming support or roaming pair. Avoid leaving squishy carries alone if they don't have an escape skill!

Earth Spirit = heralds a WTF (just kidding!)

Disruptor + Axe / Undying = very mean "cage match" with Helix or Tombstone. Sometimes done as an offlane duo, or as part of aggro tri-lane. Jakiro also combos really well with this. Countered by heroes with escapes or healing supports.Centaur / Shadow Shaman + Skywrath Mage = good control and good damage; very potent roaming pairClockwerk + Skywrath = put enemies in a box and nuke them, while interrupting them constantly.

Warlock + Undying/Elder Titan/Kunkka/Phoenix = it's bad when you're in a massive pool of syrup and can't get out of the way. Add Pugna with Nether Ward for additional enemy frustration. It's a good idea to pick another support with a teamfight ult so that the enemy has to choose between targets to interrupt. See also Team Ideas around Delayed AOE.

Anti-mage or Phantom Assassin = may prompt a last-pick Magnus, for the "free Battlefury" from Empower.